In receiver systems for radio signals containing digital components, an I/Q mixer is used to break the received signals down into their complex components, convert them to an intermediate frequency and then supply them to a demodulator unit for conversion to a demodulated signal. The demodulated signal is processed further in various circuits and is finally supplied to a circuit which converts it to a binary logic state.
In one design of an FSK receiver, the demodulator unit is in the form of a quadricorrelator. Said demodulator unit uses the complex I and Q signals applied to its inputs as a basis for producing a sequence of pulses having pulse amplitudes which are positive or negative with respect to a reference. A plurality of these pulses are averaged in an analog or digital filter, and the averaged signal is supplied to an evaluation unit.
Various circuit blocks are involved in forming the pulses within the quadricorrelator. The individual circuit blocks are not ideally matched to one another on account of component variations and mismatch. This results, at the output of the quadricorrelator, in pulses which do not have a uniform shape and, for example, have amplitudes of differing magnitude or have different zero points. Since the mismatch between the individual circuit blocks within the quadricorrelator is constant, the non-uniform shape of the pulses as well as the amplitude modulation of the pulses are repeated at regular intervals. This results in discrete components at low frequencies in the spectrum, which may interfere with the wanted signal. This interference becomes apparent from increased jitter in the demodulated signal and gives rise to errors during subsequent signal processing.